Nutrition 330 Introductory Nutrition

Study Guide: Unit 4

Carbohydrates: Sugar, Starch, and Fibre

Carbohydrates are the most abundant and readily available nutrient in foods. Intakes vary with cultural and economic factors, ranging from about 45–50% of energy consumption in the diets of people in affluent Western countries to as high as 80% in diets of people in some developing countries.

The most important function of carbohydrates is to supply energy to the body. Pure carbohydrates, such as sugar and starch, provide the same amount of energy to the body as protein: four kcalories per gram, while fat provides nine kcalories per gram. Many food sources of complex carbohydrates are also rich in protein, vitamins, and minerals. Most types of dietary fibre, which are associated with many health benefits, are also carbohydrates.

In this unit, we examine the chemical composition and classification of carbohydrates and give examples of food sources of each type. We review the processes of digestion and absorption of carbohydrates, relating them to the problem of lactose intolerance. We then discuss the functions of carbohydrates, the health effects of dietary fibre, and the regulation of blood glucose. Finally, we look at the trends in carbohydrate consumption and the Canadian recommendations for carbohydrate intake.

This unit consists of five sections:

4.1—Chemistry and Classification
4.2—Digestion and Absorption
4.3—Functions of Carbohydrates
4.4—Regulation of Blood Glucose
4.5—Carbohydrates in the Diet

Objectives

After completing this unit, you should be able to

1. define carbohydrate and identify the basic chemical components of carbohydrates.
2. identify and describe the simple carbohydrates (monosaccharides and disaccharides) and complex carbohydrates (glycogen, starch, and fibre); and give examples of food sources for each type of fibre.
3. outline the steps of carbohydrate digestion by identifying the sites of carbohydrate digestion, the substrates, the enzymes, and the products of digestion; and identify the mechanisms used in the absorption of carbohydrates.
4. define lactose intolerance, describe its symptoms, and describe dietary management to control lactose intake.
5. list and describe the functions of carbohydrates in the body.
6. describe how the body maintains and regulates its blood glucose level by the action of specific hormones.
7. describe recent trends in carbohydrate consumption in Canada.
8. discuss the health effects of excessive sugar intake and the recommendations for sugar intake.
9. discuss the health benefits of dietary fibre and recommendations for dietary fibre intake.
10. discuss the microbiome and its possible impact on health.

4.1 Chemistry and Classification

Introduction

Glucose is the principal building block of carbohydrates. During photosynthesis, plants convert carbon dioxide and water into carbohydrates (glucose) in a process requiring sunlight and chlorophyll. The carbohydrates are present in various forms. The most abundant are starch, simple sugars, and cellulose. Carbohydrates are found in all plant foods. The only animal product that contains a significant amount of carbohydrate is milk.

Carbohydrates can be classified into three main categories: simple carbohydrates, complex carbohydrates, and dietary fibre.

Objectives

After completing this section, you should be able to

• define carbohydrate and identify the basic chemical components of carbohydrates.
• identify and describe the simple carbohydrates (monosaccharides and disaccharides) and complex carbohydrates (glycogen, starch, and fibre) and give examples of food sources for each type of fibre.

Key Terms

After completing section 4.1, you should be able to define and use the following terms in context:

Reading Assignment

• Chapter 4: Introduction and “The Chemist’s View of Carbohydrates,” pages 97–103

Carbohydrate Chemistry

Carbohydrate is derived from two words: carbon and hydro (with water). Carbohydrates contain carbon, hydrogen, and oxygen atoms in the defined proportion of C(H₂O)—that is, one part carbon to one part water. The general chemical formula for the basic unit of carbohydrates—glucose—is C₆H₁₂O₆. This monosaccharide is a major building block of all carbohydrates. Other monosaccharides occurring in food are fructose and galactose. Glucose, fructose, and galactose have the same chemical formula, with only slight variation in the positioning of some atoms. These minor differences account for the contrasts in physical properties and sweetness.

You are not required to memorize any chemical structures for this course, but you should know the component atoms (C, H, O) and the general chemical formula (C₆H₁₂O₆) for a simple carbohydrate molecule. Also, you should remember that a carbon can bond with four atoms, nitrogen with three, oxygen with two, and hydrogen with one. Carbon, nitrogen, oxygen, and hydrogen are the four major elements found in nutrients. Nitrogen occurs only in proteins and B vitamins.

Simple and Complex Carbohydrates

Terms:

• Simple sugars and simple carbohydrates can be used interchangeably. Included are all monosaccharides (glucose, fructose, and galactose) and all disaccharides (sucrose, maltose, and lactose).
• Blood sugar refers to glucose in the bloodstream.
• Dextrose is a term commonly used in food labels; it refers to glucose.
• Dextrin is a short chain of glucose units resulting from the partial hydrolysis of starch. It is often used as a thickening agent in foods.
• Refined sugars are such sugars as white table sugar or corn syrup. They contain virtually pure simple carbohydrates, and are often called empty‑calorie (that is, empty of nutrients other than energy) foods.
• Refined cereals are grains that have been milled to remove the bran and germ, leaving only the endosperm. Examples are white flour, polished (white) rice, and cornstarch. Refined cereals consist mainly of starch and have a much lower content of dietary fibre, vitamins, and minerals than unrefined (whole-grain) cereals. This was discussed in the textbook (p. 48–50). Refined carbohydrates include refined sugars and refined cereals.
• Complex carbohydrates include any carbohydrate consisting of more than two units of glucose; complex carbohydrates are also called polysaccharides. This group includes glycogen, starches, and dietary fibre.

As your textbook explains, all carbohydrates (as well as proteins and fats—Units 5 and 6), are put together and taken apart by similar chemical reactions: condensation and hydrolysis. Figures 4–6 and 4–7 (p. 100) illustrate these two reactions.

Dietary fibre is discussed in detail later in this unit.

Study Questions

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4.2 Digestion and Absorption

Introduction

When one consumes carbohydrate‑containing foods, the body must digest the carbohydrates into the basic units of glucose, fructose, and galactose. Only monosaccharides can be absorbed by the intestinal mucosal cells. Absorbed fructose and galactose are converted to glucose in the liver and possibly also in the intestinal mucosal cells. Ultimately, glucose will be circulated in the bloodstream to the liver and other parts of the body. Once inside a cell, glucose is metabolized according to body needs. It can be stored as glycogen or fat, used to produce other body compounds, or metabolized to produce energy, carbon dioxide, and water.

Objectives

After completing this section, you should be able to

• outline the steps of carbohydrate digestion by identifying the sites of carbohydrate digestion, the substrates, the enzymes, and the products of digestion; and identify the mechanisms used in the absorption of carbohydrates.
• define lactose intolerance, describe its symptoms, and describe dietary management to control lactose intake.

Key Terms

After completing section 4.2, you should be able to define and use the following terms in context:

Reading Assignment

• Chapter 4: “Digestion and Absorption of Carbohydrates,” and “Lactose Intolerance,” pages 103–107

Carbohydrate Digestion

Figure 4‑10 (p. 104) of the textbook clearly outlines the steps of carbohydrate digestion in the GI tract.

Carbohydrates are absorbed in the form of monosaccharides—primarily glucose and fructose—plus a very small amount of galactose. Absorption takes place mainly in the intestinal mucosa of the jejunum and the ileum, although some glucose can be absorbed through the lining of the mouth.

Lactose Intolerance

The textbook presents a concise discussion of lactose intolerance. In many cases, people with lactose intolerance can tolerate some lactose without symptoms. Foods such as cheese, which contains little lactose, often does not pose any problems. However, the amount of lactose in cottage cheese may vary because milk solids are an optional ingredient in the creaming mixture of cottage cheese. Yogourt contains lactose, but most of it degrades to lactic acid during fermentation by the bacterial culture, so it can often be tolerated by people with lactose intolerance.

Study Questions

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4.3 Functions of Carbohydrates

Introduction

The primary function of carbohydrates is energy. The body tissues require a constant supply of glucose for all metabolic functions. Body storage of carbohydrates is relatively limited; only about 110 grams of glycogen are stored in the liver and about 225 grams in the muscles, giving a total storage of about 335 grams, plus about 10 grams as blood glucose. Each gram of carbohydrates yields four kilocalories. This total glycogen storage—equivalent to about 1,340 kilocalories—would not meet most people’s daily energy requirement. Therefore, people must consume carbohydrates daily to meet their energy needs and to maintain a sufficient glycogen storage.

Only an overview of some metabolic processes will be given here. A more detailed explanation will be presented in later units.

Objective

After completing this section, you should be able to list and describe the functions of carbohydrates in the body.

Key Terms

After completing section 4.3, you should be able to define and use the following terms in context:

ketones
ketosis
gluconeogenesis

Reading Assignment

• Chapter 4: “Glucose in the Body,” pages 107–108 (up to “The Constancy of Blood Glucose”)

Functions of Carbohydrates

The primary function of carbohydrate is to supply glucose. Glucose is distributed to all body cells via the bloodstream and used for energy. Optimal functioning of the body cells requires a normal range of blood glucose. Signs of low blood sugar are tiredness, shakiness, and hunger. Signs of high blood sugar are sleepiness, frequent urination, and thirst. In extreme cases, blood sugar levels that are above or below the normal range results in coma. Generally, in a healthy individual, the body can regulate the blood glucose level within the normal range. Glucose regulation will be discussed in the next section.

Carbohydrates are essential to the normal functioning of some specialized body cells, including those of the nerve tissue of the brain and of the central nervous system, the lungs, and the red blood cells. These cells require glucose as their sole energy source. In prolonged fasting, some brain cells partially adapt by using a small amount of ketones (intermediate products in the breakdown of fats) to fuel the brain. However, other brain cells still rely exclusively on glucose.

Carbohydrates “spare” protein. In the body, energy needs take precedence over all others. Body proteins can be converted to glucose (gluconeogenesis) to provide energy when there is insufficient carbohydrate. However, the primary function of proteins in the body is for synthesis and maintenance of body tissues and fluids. To allow optimal functioning of the proteins, sufficient carbohydrates are needed to spare proteins from being used for energy. Even in low‑calorie diets, enough carbohydrates should be included to prevent muscle loss.

Carbohydrates prevent ketosis. When there is insufficient carbohydrate, larger amounts of fats are broken down for energy—more than the body is equipped to handle. As a result, fats are incompletely oxidized, which produces ketones. When ketone bodies are not excreted rapidly enough, they accumulate, causing ketosis. We will discuss ketosis in Unit 7 when we study the metabolism of nutrients.

Carbohydrates help to maintain the body’s normal balance of water and sodium. As the body tries to excrete ketone bodies produced during the breakdown of fats, sodium is excreted as well, because sodium binds with acidic ketones, which are excreted as sodium salts in the urine. Since sodium and water are excreted together, the loss of sodium automatically leads to loss of body fluid, resulting in dehydration and sodium imbalance. These symptoms, plus fatigue and muscle wasting, are typical in people consuming low-carbohydrate diets. This fluid loss partly explains why low‑carbohydrate diets cause rapid weight loss. An additional reason low‑carbohydrate diets cause rapid weight loss is because one gram of stored glycogen holds four grams of water. Thus, the 335 grams of glycogen that can be stored in the body will release about 1.3 litres (1.3 kg or 2.9 lb) of water if these stores are depleted due to lack of ingested carbohydrate. However, most of this weight returns when carbohydrate intake is normalized.

Carbohydrates can combine with protein and lipid molecules to produce vital body components, including glycoproteins in mucous secretions and connective tissues, and glycolipids in brain and nerve tissues. Also, glucose is a precursor of pentoses (5‑carbon sugars), which are required for the synthesis of the nucleic acids, DNA and RNA.

Study Questions

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4.4 Regulation of Blood Glucose

Introduction

Blood glucose levels are normally maintained within a narrow range. This homeostasis is maintained and controlled by several hormones in the body: insulin, glucagon, and epinephrine.

Objective

After completing this section, you should be able to describe how the body maintains and regulates its blood glucose level by the action of specific hormones.

Key Terms

After completing section 4.4, you should be able to define and use the following terms in context:

insulin
glucagon
epinephrine (adrenalin)

Reading Assignment

• Chapter 4: “The Constancy of Blood Glucose,” pages 108–112

Hormonal Control of Blood Glucose

Normally, blood glucose levels are maintained within a narrow range. Table 4.1 outlines the ideal ranges of blood glucose levels at different intervals before and after food consumption (for information only).

Table 4.1 Blood Glucose Levels (mmol/L)

Homeostasis of blood glucose is maintained and controlled by several hormones in the body.

Insulin: After a meal, the blood glucose level rises. The pancreas (beta cells of islets of Langerhans) secretes insulin to reduce the blood glucose level. Insulin also stimulates fat cells to take up and store fat. Insulin reduces blood glucose by

• increasing the permeability of cell membranes, allowing glucose to enter the cells;
• stimulating the production of energy from glucose;
• facilitating the conversion of glucose to glycogen in muscle and liver cells, and
• facilitating the conversion of glucose to fat in liver cells.

Glucagon: Between meals, the blood glucose level drops. The pancreas (alpha cells of the islets of Langerhans) secretes glucagon to increase the blood glucose level by

• facilitating the conversion of glycogen stored in the liver to glucose, which enters the bloodstream and circulates to the body, and
• stimulating the conversion of body protein to glucose (gluconeogenesis) and energy if the glycogen stores in the liver are exhausted.

Epinephrine (adrenalin): During “fight or flight” (stress) situations, epinephrine is secreted to ensure that sufficient energy is available. It stimulates the conversion of liver and muscle glycogen to glucose and energy. Epinephrine works similarly to glucagon, but it is stimulated by different circumstances.

To help the body regulate blood glucose, it is best to avoid extremes of food intake by

• eating when one is hungry, not waiting until one is famished. In our busy lives, people often skip meals.
• eating balanced meals, with a combination of protein and complex carbohydrate. Protein stimulates glucagon to counteract the effect of excessive insulin release, so that the blood glucose level stays within the normal range for a longer period of time. Hence, small, frequent intakes of protein is recommended to help maintain the blood glucose level, especially for people who tend to be hypoglycemic. Fibre and fat help to slow the rate of digestion and absorption of food, which also helps regulate the entry of glucose into the bloodstream.
• spreading food intake evenly over three or four meals a day and avoiding overconsumption at any one meal.

Study Questions

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4.5 Carbohydrates in the Diet

Introduction

Lifestyle changes affect daily food choices and nutrient intakes. Large increases in sugar consumption are among the significant changes in carbohydrate intake. Dietary fibre has also attracted much attention from researchers, nutritionists, and the general public.

Objectives

After completing this section, you should be able to

• describe recent trends in carbohydrate consumption in Canada.
• discuss the health effects of excessive sugar intake and the recommendations for sugar intake.
• discuss the health benefits of dietary fibre and recommendations for dietary fibre intake.

Key Terms

After completing section 4.5, you should be able to define and use the following terms in context:

Determining Nutrient Consumption

The average consumption of nutrients in a country is often measured as the amount of the nutrient that disappears from the marketplace per person per year, calculated as

per capita disappearance per year=total production+imports−exportstotal population

This measurement overestimates consumption, because not all food produced is eaten: some spoils, some is wasted, and some is not used for human consumption. Disappearance data are estimated to inflate actual consumption by 25–30%.

Another method used to estimate average consumption of nutrients is to measure the quantities of food bought weekly by families for at‑home consumption and to divide that by the family size and by seven days. Meals eaten away from home are also considered. Consumption measured in this way may also be overestimated because it doesn’t account for table waste and discarded leftovers. Statistics Canada uses this method every two years or so to compile the “Family Expenditure Survey Data.”

Changing Sources of Carbohydrate

Since the 1950s, significant changes have occurred in Canadian lifestyles. For example, more women are working outside the home, making more disposable income available in many families; more meals are consumed away from home; and more convenience foods are eaten. All of these changes have had an impact on our daily food choices and nutrient intakes.

A major dietary survey revealed that, on average, Canadians consume approximately 49% of their energy from carbohydrate (Kirkpatrick and Tarasuk, 2008).

Since the 1970s, the sources of carbohydrates in our diet have changed: we are consuming more rice and legumes. There has also been a small increase in the consumption of wheat flour (Statistics Canada, 2009), but very little change in the amount of potatoes eaten. Fruit and vegetable consumption (apart from potatoes) have both risen modestly, while consumption of fruit juice has doubled. The most dramatic change over the last four decades has been the major increase in consumption of soft drinks. Overall, about half of the total carbohydrate intake is in the form of complex carbohydrates, mainly starch from vegetables and cereals.

Based on disappearance data, sugar consumption per capita in Canada indicates an average of about 41 kg (91 lb) per person per year. Since figures of actual consumption are usually 25–30% lower, it is estimated that the average person consumes about 30 kg (67 lb) of sugar per year (about 14% of the food energy supply). The dominant sweetener used in Canada is sugar (sucrose). Canadians also consume considerable amounts of high‑fructose corn syrup (HFCS), especially in soft drinks.

HFCS has attracted much attention. There has been much speculation that it is somehow more harmful than sucrose. HFCS refers to sugar extracted from corn. Its fructose content is usually either 42 or 55%, with the remainder being glucose. These rather small differences compared with sucrose appear to have minimal effects on risk of obesity or on any other health condition.

Reading Assignment

• Chapter 4: “Health Effects and Recommended Intakes of Sugars,” pages 112–116

Health Effects of Sugar Consumption

The controversies stemming from sugar consumption have triggered much concern and initiated many research studies and task forces. Because sugar is devoid of vitamins and minerals, excessive intake of sugar may lower the intake of these nutrients in the diet overall. It has been well known for decades that sugar plays an important role in tooth decay, particularly when the sugar is eaten in a sticky form, as in candy. Sugar also has a poor ability to satisfy the appetite. Consequently, it encourages overeating and excess weight gain.

A great many studies have been conducted in recent years that shine light on the relationship between sugar intake and other aspects of health. In the last few years, important new findings have emerged that directly link sugar with risk of both type 2 diabetes (Malik et al., 2010) and heart disease (Fung et al., 2009). The strongest evidence pointing to sugar as a causative factor in obesity, diabetes, and heart disease is seen for sugar‑sweetened beverages, such as cola drinks. The textbook fails to mention the connection between sugar and risk of diabetes and heart disease.

Therefore, sugar consumption should ideally be reduced from present levels. A cola drink contains about 11 or 12 grams of sugar per 100 mL, which means that a can (333 mL) delivers about 36 grams or 145 kcal of sugar. Thus, an adult with a typical energy intake (2000–2500 kcal per day) who drinks a can of cola every day will obtain about 5–7% of his or her energy from this one source. The textbook (bottom of p. 115) suggests that intake of sugar should be limited to 5–10% of energy intake. That means that one can of cola (or the equivalent amount of sugar from other sources) should be the upper limit of daily intake for most people.

Reading Assignment

• Chapter 4: “Fibres,” page 102
• Chaper 4: “Health Effects and Recommended Intakes of Starch and Fibres,” pages 119–122

Recommendations for Carbohydrate Intake

About 180 grams of carbohydrates per day is needed to permit complete oxidation of fats and to avoid production of ketone bodies. About 130 grams can be supplied by the body through gluconeogenesis (synthesis of glucose from non‑carbohydrate sources, e.g., protein). Therefore, an intake of at least 50 grams of carbohydrates per day is required for normal functioning of the body. The RDA for carbohydrate is 130 grams per day, based on the average minimum amount of glucose needed by the brain.

The Acceptable Macronutrient Distribution Range (AMDR) for carbohydrate is 45–65% of total kcalories, with most coming from complex carbohydrates. A diet high in complex carbohydrates is generally low in fats and high in dietary fibre; such a diet has been associated with a low incidence of heart disease, obesity, and colon cancer.

Dietary Fibre

Dietary fibre has attracted much attention from researchers and nutritionists, as well as from the general public. Epidemiological data have indicated correlations between high‑fibre foods and a low incidence of diseases such as cardiovascular disease, colon cancer, diverticular disease, appendicitis, hemorrhoids, and obesity. However, such data cannot prove that a low fibre intake causes these diseases; other dietary and lifestyle factors that might contribute are a low intake of vegetables, smoking, and a sedentary lifestyle. The only disorder for which there is general agreement that dietary fibre does play a preventive role is constipation, (i.e., a low fibre intake is proven to be a major cause).

Nevertheless, the textbook outlines several beneficial effects of fibre intake and relates different types of fibre to possible prevention or treatment of some diseases (see Table 4‑3 on page 121). You should be able to describe the physiological effects and health benefits of soluble and insoluble fibres, and to give some examples of dietary sources. A third type of fibre, known as resistant starch, is now being recognized as potentially significant to colonic health. Fermentation of resistant starches as well as soluble fibre results in the production of short‑chain fatty acids, which may inhibit growth of cancer cells and reduce inflammation. Legumes are particularly good sources of these resistant starches. Canada’s Food Guide specifically recommends eating beans and legumes often. Thus, it is important not only to include a sufficient quantity of fibre in the diet, but also to obtain fibre from a wide variety of food sources.

As with any nutrient, excessive intake of fibre may be undesirable, particularly insoluble fibre, which can interfere with absorption of minerals like calcium, copper, magnesium, iron, and zinc, and possibly some vitamins. However, the risk of excessive intake of fibre is low.

Your text discusses a variety of recommendations for fibre intake. The DRI for fibre, based on energy intake, is 14 grams per 1000 kcalories. This translates to about 21 to 38 grams per day, depending on age and gender. Daily fibre intakes in Canada are much lower than the recommended intake, ranging from 17–21 grams for men and 10–18 grams for women. Canada’s Food Guide addresses this by placing emphasis on whole grains, vegetables and fruits, and meat alternatives like legumes and beans.

The Microbiome

There has been great interest in recent years in the activity of the intestinal bacteria and other microorganisms. The microbiome, as it is now known, is believed to be strongly involved in many disease processes, either in a beneficial or harmful way.

The microbiome is the overall microbial ecosystem in the human body and is found mainly in the gastrointestinal tract (GIT). It consists of hundreds of species of bacteria, yeast, fungi, and viruses. It can confer many positive health benefits such as protecting the GIT, aiding in digestion, and in the production of short‑chain fatty acids and some vitamins. The microbiome directly or indirectly influences other organ systems through modulation of hormones, immunity, and even brain function. The role of the microbiome in immunity influences susceptibility to a variety of diseases.

Nutrition influences the microbiome by providing nutrients. A healthy diet is vital for helping to maintain a healthy microbiome. A substance that enhances the growth of beneficial microorganisms is known as a prebiotic. Many types of dietary fibre can act as a prebiotic. A popular type of dietary supplement are probiotics. These are live microorganisms sold as pills. Probiotics can also be obtained from fermented foods, particularly dairy products, such as yogourt.

A healthy microbiome can be disrupted by antibiotics, diet, or other stresses. This may result in damage to the mucin layers, which protect epithelial cells. This can allow pathogenic microorganisms to cross the mucosal barrier into the body and cause illness or disease.

Future research in nutrition and the microbiome will undoubtedly add to our understanding of the influence of the microbiome on health and disease.

Study Questions

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References